I can scan these networks only by scanning 192.168.5.1 - 192.168.113.254; it's a bit... long. I've tried to scan "192.168.5.1 - 192.168.5.254 ; 192.168.113.1 - 192.168.113.254", but this range is not allowed on WiFi Guard.

The short answer: WiFi Guard cannot determine MAC addresses outside the current subnet.

The long answer: The ARP protocol that WiFi Guard uses to find out the MAC address of a device does not work across subnets. In your case the 192.168.2.x network is behind the router, so there is no way to discover MAC addresses in that network. As the sole purpose of WiFi Guard is to identify devices by MAC address, it does not show any devices whose MAC address cannot be determined.